Recently, in the field of endoscopes, a swallowable capsule endoscope including an imaging function and a radio communication function has been proposed. Development of an in-vivo information acquiring system for acquiring image data on an intra-subject image picked up by the capsule endoscope is underway. In this in-vivo information acquiring system, the capsule endoscope moves in a subject's organ, e.g., the stomach or the small intestine, according to the peristaltic movement of the organ until natural discharge of the capsule endoscope after the subject, e.g., patient swallows the capsule endoscope from his/her mouth. Furthermore, the capsule endoscope functions to pick up intra-subject images at predetermined intervals, e.g., 0.5-second intervals.
During the movement of the capsule endoscope in the body of the subject, the image data obtained by the capsule endoscope is sequentially transmitted to the outside by radio communication, and received by a receiving apparatus through receiving antennas distributed outside of the subject. The receiving apparatus demodulates a radio signal received through the receiving antennas into an image signal, performs a predetermined image processing on the obtained image signal, and generates image data. Thereafter, the receiving apparatus sequentially stores image data thus generated (that is, information on images picked up by the capsule endoscope) in a storage medium. By causing the subject to carry such a receiving apparatus including the radio communication function and a memory function, the subject can move freely until the discharge of the capsule endoscope after the subject swallows the capsule endoscope. A user, e.g., a doctor or a nurse, loads the image data stored in the storage medium of the receiving apparatus into a workstation, and diagnoses the subject while displaying the intra-subject images on a display of the workstation (see, for example, Patent Document 1).
Generally, a plurality of receiving antennas of the receiving apparatus for receiving the radio signals transmitted from the capsule endoscope are arranged to be distributed outside of the subject's body. Furthermore, the receiving apparatus selects and switches to one receiving antenna with fewer radio-signal reception errors and receives the radio signals through the selected receiving antenna.    Patent Document 1: Japanese Patent Application Laid-Open No. 2003-19111